This research is concerned with mechanisms of opioid modulation of behavior during development. I propose to investigate a number of phenomena that we have discovered during our most recent finding period. They are: (1) The palliation of stress and pain, through opioid mediation, by sweet and other palatable solutions. (2) The interactions between opioid systems and cholecystokinin as they influence distress reactions. (3) The implications of delayed tolerance in rats on opioid-mediated behaviors. (4) The contribution of opioids to the formation of mother-infant bonds and (5) The development of a model for assessing how maternal chocoholism during pregnancy and lactation influence, behavior of suckling rata before and after the onset of tolerance mechanisms. This last aim arises from findings obtained in our research of human infants. Together, the proposed studies will reveal and explore the characteristics of different interactions between opioid, behavioral, and physiological processes during ontogeny.